1. Field of the Invention
The present invention relates to an optical recording medium including a recording layer having a semi-transparent structure, a method of recording information on the optical recording medium, and an information recording and reproducing apparatus.
2. Description of the Related Art
There is, for example, a so-called phase-change double-layered optical recording medium including two recording layers, each being formed of a phase-change recording layer, i.e., an L0-layer (a rear-side layer) and an L1-layer (a front-side layer).
In an optical recording medium such as a DVD (Digital Versatile Disc), data to be recorded is modulated to have a length of a recording mark along a track of a recording layer. At the same time, the recording is performed while the length of the recording mark is made to correspond to an integral multiple nT of T where T is one clock cycle. For example, in a CD-RW (compact disc rewritable), an EFM modulation technique is used; data to be recorded is modulated to a recording mark having any of lengths 3T to 11T so as to record the data.
In a rewritable optical recording medium such as the above-mentioned CD-RW, a phase-change layer is generally used as a recording layer. After pulse-modulated in accordance with information to be recorded, laser beam pulses are irradiated on a recording layer so as to form recording marks having a length corresponding to nT. In the recording mark area, the phase-change layer is converted to an amorphous state. Between the recording marks, a space area in a crystalline state is created.
The recording mark area in the amorphous state is formed by melting a crystalline part with a laser beam and then rapidly cooling it. The crystalline part is formed by irradiating laser beams so as to keep a crystallizing temperature of the phase-change layer or higher for a given time or longer.
In the case where the amorphous state is to be obtained, a pulse series of laser beam composed of a write pulse at a recording power and a cooling pulse at an approximately base power is irradiated so as to melt the recording mark area followed by rapid cooling. In the case where the crystalline state is to be obtained, a laser beam at an erase power level is irradiated.
For example, in the above-mentioned CD-RW, a pulse series of laser beam composed of (n−1) write pulses and cooling pulses is irradiated to create an nT recording mark. For example, in order to create a 5T recording mark, a pulse series of laser beam composed of 4 (=5−1) write pulses and cooling pulses is irradiated.
In particular, if a mark length is increased by 1T, for example, from 5T to 6T, the mark length is controlled by adding a pulse series composed of a write pulse and a cooling pulse for a 1T interval.
On the other hand, with the increased density of optical recording media, a multi-layered optical recording medium including a plurality of recording layers has been proposed. A light incident-side recording layer thereof is required to have a semi-transparent structure with a light transmittance of 30 to 80% at a recording wavelength so as to enable the recording and reproduction of data on and from a bottom-side recording layer. Since the optical recording medium including a phase-change recording layer uses a metal heat-sink layer and includes the light incident-side recording layer having a semi-transparent structure, a thickness of the metal heat-sink layer is required to be reduced to, for example, less than 30 nm, which was conventionally about 100 nm. With the reduced thickness of the metal heat-sink layer, a heat release effect is also reduced. As a result, the light incident side-recording layer has a slow cooling structure.
In the recording layer having a slow cooling structure, a part of the area, which was conventionally converted to an amorphous state by rapid cooling, cannot be rapidly cooled. Therefore, there arises a problem of thermal interference between the adjacent recording marks or thermal interference between tracks, called cross-erase.
The thermal interference between the recording marks or the cross-erase between the tracks as described above is a problem not only in multi-layered optical recording media but also in single-layered optical recording media. Therefore, for example, countermeasures as described in Japanese Patent Laid-Open Publications Nos. 2003-203337 and 2003-208713 have been proposed.
In Japanese Patent Laid-Open Publication No. 2003-203337, an erase power level is changed in accordance with the space length of the area to be erased. In Japanese Patent Laid-Open Publication No. 2003-208713, setting two power levels for a write pulse has been proposed.
However, any of the above countermeasures increases the number of power levels of a laser pulse, inducing another problem that the modulation of laser becomes difficult.
Furthermore, with the increase in recording speed, a recording strategy for synchronizing a laser pulse with a 2T clock (see Japanese Patent No. 3508932) and a recording strategy using the combination of 1.5T to 3T clocks (see Japanese Patent Laid-Open Publication No. 2002-312934) have been proposed. According to these strategies, however, a write pulse width tends to increase (become wider) in proportion to a length of a clock cycle because the write pulse width with respect to the clock cycle with which the laser pulse is synchronized is not taken into consideration. Since the write pulse width is long (wide) in these strategies, a melt range during recording is expanded in a track width direction to increase the cross-erase. In particular, the recording layer having a slow cooling structure has a problem that the cross-erase becomes noticeable.